The present invention relates to a quantum circuit device for computing by using quantum dots.
The quantum computer and quantum circuit were invented by D. Deutsch (refer to, e.g., D. Deutsch, Proc. R. Soc. London, Ser. A 400, 97 (1985); 425, 73 (1989)). Thereafter, discoveries of effective quantum algorithms for factorization into prime factors, searches, etc. have induced to propose a number of physical systems. Actual operations of some of the physical systems which are circuits of merely few qubits have been confirmed. However, these systems could be integrated up to 10 qubits at most because of restrictions, as of decoherence and hardware number increase, etc. Usable operations require systems of 100 to 1000 qubits classes.
The quantum computer is never used alone, and is not operated and does not exhibit its ability without being closely connected to classic computers. This is evident when the algorism and the protocol for error corrections of, e.g., P. W. Shor (refer to, e.g., P. W. Shor, Proceedings of the 35th Annual Symposium on Foundations of Computer Science, edited by Goldwasser (IEEE Computer Society, Los Alamitos, Calif., 1994), p. 124). In consideration of the integration and the interface with the classic computer, it is preferable that the quantum computer is of course based on semiconductors.
Then, B. E. Kane proposed that the nuclear spin of P (phosphorus) which is a donor in a bulk 28Si is used as a qubit (refer to, e.g., B. E. Kane, Nature (London) 393, 133 (1998)). It is known that such nuclear spins isolated from the environment keep coherence for a very long time. However, this mode requires growth of isotope silicon crystals of high purity and control of precise location of individual impurity ions. A more realistic approach will be a semiconductor nanostructure. However, in the nanostructure as well, because of continuous state densities in structures other than the zero-dimensional structure, energy relaxation easily takes place, and accordingly this mode is difficult to realize. Fortunately today, the zero-dimensional structure, i.e., quantum dots can be formed by various methods.
In such backgrounds, A. Barenco et al. proposed qubits by using quantum dots (refer to, e.g., A. Barenco, D. Deutsch, A. Ekert, and R. Jozsa, Phys. Rev. Lett. 74, 4083 (1995)). In this mode, quantum energy levels in the quantum dot are used as 2 states of the qubit. Coulomb interactions between dipole moments of two adjacent quantum dots, which are induced by an external electric field are used in the two-qubit computation. Experimentally as well, observation of coherent quantum levels of quantum dots and control of electron numbers of quantum dots by using a single electron effect are possible. However, generally, electrons of an excited level are strongly coupled to electromagnetic environments and acoustic environments, and have strong decoherence.
The established quantum error correction and quantum fault-tolerance theories have made clear that the decoherence is tolerable to some extent (refer to P. W. Shor, Phys. Rev. A 52, R2493 (1995), etc.). However, to this end, the decoherence must be below a certain criterion. The simple quantum dot-type qubit of the mode proposed by A. Barenco have found it difficult to clear the criterion.
As means of solving the above-described problems, D. Loss and D. P. DiVincenzo have proposed a mode using a spin of electron in a quantum dot (refer to, e.g., D. Loss and D. P. DiVincenzo, Phys. Rev. A 57, 120 (1998)). Spins are expected to be less strongly coupled to environments than charges. They use, for the two-qubit operation, swap operation of coupled quantum dots in placed of the usually used controlled NOT operations. This is virtually a two-electron quantum beat; a barrier between two adjacent quantum dots is made higher and lower to permit the tunnel, whereby the beats are switched on. However, their mode has a problem in means for making the barrier between the quantum dots higher and lower. The method of switching external magnetic fields has a limit to operational speeds. No high-level processing technique for a method of applying an electric field by a micro-electrode to thereby control a potential of the barrier has been so far established.
As described above, for the quantum circuits characterized by the two-quantum level systems as operational ground states, the constitutions using quantum level or spin of electron in the two-level systems have been conventionally shown. However, the former can relatively easily make operations by using laser pulses while having high decoherence due to relaxation between the levels. On the other hand, the latter has a merit that decoherence of spins is low while control of gate operations is difficult. To be more specific with the latter, although gates must be quickly on/off to make operations precise, the use of magnetic fields makes the high speed switching very difficult. Thus, the quantum circuit devices for executing quantum operations have various problems to be solved.
An object of the present invention is to provide a quantum circuit device which can operate relatively stably at high speed.
According to one aspect of the present invention, there is provided a quantum circuit device comprising: a first asymmetrical coupled quantum dot including: a first main quantum dot; and a first operational quantum dot of a smaller size than that of the first main quantum dot, which are coupled to each other; a second asymmetrical coupled quantum dot including: a second main quantum dot arranged at a distance which does not substantially permit tunneling from the first main quantum dot; and a second operational quantum dot having a smaller size than that of the second main quantum dot and arranged at a distance which permits tunneling from the first operational quantum dot, which are coupled to each other; and a laser device for applying to the first and the second asymmetrical coupled quantum dots a laser beam of a wavelength which resonates an inter-level energy with respect to the first and the second asymmetrical coupled quantum dots.
According to another aspect of the present invention, there is provided a method of quantum operation for a quantum circuit device comprising: a first asymmetrical coupled quantum dot including: a first main quantum dot; and a first operational quantum dot of a smaller size than that of the first main quantum dot, which are coupled to each other; and a second asymmetrical coupled quantum dot including: a second main quantum dot arranged at a distance which does not substantially permit tunneling from the first main quantum dot; and a second operational quantum dot having a smaller size than that of the second main quantum dot and arranged at a distance which permits tunneling from the first operational quantum dot, which are coupled to each other, the method comprising the step of making an operation, based on a spin of an electron by using a quantum circuit having a ground state localized substantially in the first and the second main quantum dots and an excited state which is not substantially present in the first and the second main quantum dots.
As described above, according to the present invention, operational cells each comprise an symmetrical coupled quantum dot including a main quantum dot of a larger size and an operational quantum dot of a smaller size. In the sleep state, electron is present at the ground state of the main quantum dot, where no exchange interaction takes place, and in an operation, the electron is transited to an excited state of the operational quantum dot, whereby the operation is made by the exchange interactions between the adjacent operational quantum dots. Thus, relatively stable operations can be made. The electron transition between the main quantum dot and the operational quantum dot is caused by application of laser beams, which makes the constitution very easy to be fabricated.
The ancillary quantum dot is arranged adjacent to the main quantum dot and in magnetic field, whereby the one-qubit operation can be made by using the ancillary quantum dot. The observation quantum dot is arranged adjacent to the ancillary quantum dot, and based on a state of the observation quantum dot, polarization in the main quantum dot is detected by the RF-single electron transistors, whereby state of the main quantum dot can be detected at high speed.
The laser beam source for generating xcfx80 pulse to be used in the electron transition is provided by a long wavelength laser, such as the quantum cascade laser, the narrow band gap semiconductor laser or others, whereby the quantum circuit device according to the present invention can make optimum achievements.